Don't have a link as I read this about 1-2 months ago, but Microsoft has officially stated that DX10 not be available for XP (probably due to it's full-rewrite and the current state of XP's display model).
DX10 only games won't really start to show until the end of 07' I would expect. With many not keen on upgrading to Vista, I'm sure most developers wouldn't be foolish to isolate the XP customer base. I'm real excited about Crysis.. and with the R600 to have a rumoured 64 unified shaders, SM4.0 compliant and having full DX10 compatibility.. I can't wait till November
(assuming the suggestions that their recent tape out was successful isn't false)..
G80 is rumoured to be taped-out too, although I'm less keen on nVidia's bet on a supposed 32:16:16 Pixel/Vertex/Geometry shader architecture.